How do I make a Decal look worn?
Given that you are using the alpha channel as a mask to control the mix of different elements, you need to make the mask less perfect. Instead of being a perfect white mask, you need to subtract from the white using a texture. Why subtract? Think of white as 1 and Black as 0. You want to affect the mask without adding any more information to the black areas. 

 

Subtract a texture from the alpha channel, and use the result as a Mask to control the mix for the color for the shader.



Combining other textures and subtracting them from the alpha you can further distress a texture to seem more random.